"Doctrine" is a synonym for "teaching", as indicated by the fact that the word is derived from the Latin verb "docere", which means "to teach".
As indicated on the website http://www.biblehub.com, the original Greek word in the cited verse was (in Roman characters) "didachon", which can be seen to be the linguistic root for the word "didactic". Aside from any negative connotations that that word may have developed over time, it originally likewise indicated an association with teaching.
In short, therefore, the apostles' "doctrine" would have referred to their teaching, which, in turn, would have consisted of informing new or potential Christians about Jesus -- not just from the standpoint of what He had taught, but also focusing on the necessity of faith in His atoning death and subsequent resurrection as the only means to receive forgiveness of sins and eternal life. (These were the same points that Peter had just stressed in his address at Pentecost -- as indicated in Acts 2:38-39 -- and that he later emphasized in speaking to the Sanhedrin in Acts 4:8-12.)